The present invention generally relates to communications systems and, more particularly, to satellite-based communications systems.
Generally speaking, in a satellite communications system a ground station transmits a signal “uplink” to a satellite transponder, which re-transmits the signal “downlink” to a receiving station. One form of satellite communications system employing hierarchical modulation is described in U.S. Pat. No. 5,966,412 issued Oct. 12, 1999 to Ramaswamy. Backward-compatible hierarchical modulation(BCHM) can be used in a satellite system as a way to continue to support existing legacy receivers yet also provide a growth path for offering new services. In other words, a BCEM based satellite system permits additional features, or services, to be added to the system without requiring existing users to buy new satellite receivers. In a hierarchical modulation based communications system, at least two signals, e.g., an upper layer (UL) signal and a lower layer (LL) signal, are added together to generate a synchronously modulated satellite signal for transmission. In the context of a satellite-based communications system that provides backward compatibility, the LL signal provides additional services, while the UL signal provides the legacy services, i.e., the UL signal is, in effect, the same signal that was transmitted before —thus, the satellite transmission signal can continue to evolve with no impact to users with legacy receivers. As such, a user who already has a legacy receiver can continue to use the legacy receiver until such time that the user decides to upgrade to a receiver, or box, that can recover the LL signal to provide the additional services.
In communications systems, error detection/correction codes (and interleavers) are used to improve the reliability of transmission. Such error detection/correction codes includes such techniques as, but not limited to, convolutional codes, trellis codes, a concatenated forward error correction (FEC) scheme, where a rate 1/2, 2/3, 4/5 or 6/7 convolutional code is used as an inner code, and a Reed Solomon code is used as an outer code; LDPC codes (low density parity check codes); etc. For example, in the context of the above-described hierarchical modulation based satellite system, the UL signal is typically encoded using a convolutional code or a short block code; while the LL signal is typically encoded using a turbo code or LDPC code.
In the context of a turbo code or an LDPC, the receiver typically uses an iterative decoding technique such as represented by a soft-input-soft-output (SISO) technique. SISO is typically based upon “soft metrics” such as log-likelihood ratios (LIRs). In general terms, an LLR is related to the probability that a particular received bit (binary digit) value is either a logical “one” or logical “zero.” In particular, the transmitter transmits symbols from a predefined signal space, each transmitted symbol having associated therewith a given  bits-to-symbol mapping M(bi), where M are the target symbols and bi; i=0, 1 . . . B−1 are the bits to be mapped where B is the number of bits in each symbol. For example, in a 16-QAM (quadrature amplitude modulation) signal space, there are 16 symbols, each symbol mapped to a particular four bit value(B=4). At the receiver, the received signal is processed into a stream of signal points, each signal point residing in the above-mentioned signal space (but not necessarily corresponding to a particular transmitted symbol due to noise). The receiver calculates the LLRs, i.e., the likelihood that a particular bit value was received given a received signal point. In general, the log-likelihood ratio function for the ith bit of the B bit value is calculated as follows:
                                                                                          LLR                  ⁡                                      (                                          i                      ,                      z                                        )                                                  =                                  log                  ⁡                                      [                                                                  prob                        ⁡                                                  (                                                                                    b                              i                                                        =                                                          1                              |                              z                                                                                )                                                                                            prob                        ⁡                                                  (                                                                                    b                              i                                                        =                                                          0                              |                              z                                                                                )                                                                                      ]                                                              ;                                                                          i                =                0                            ,              1              ,                                                …                  ⁢                                                                          ⁢                  B                                -                1                                                                        (        1        )            where, z is the received signal point value. Generally, if the LLR value is positive, the bit is most likely to be a 1; while if the LLR is negative, the bit is most likely to be a zero. The receiver iteratively decodes the received signal using the calculated LLRs.